What would be the best time to invade?
Well, to be an honest killjoy, there probably wasn't after the actual Korean War. Even at the biggest on-paper (and unsustainable) advantage in the late 70s through much of the 80s, the ROK would never again be the pushover it was in 1950. Whatever else he was, Kim Il-Sung was no fool, and he knew this.
If I had to pick a point, I'd say from 1975 (right after Vietnam like you said) to the mid-1980s (After the North's buildup but before the South's boom really reaches its army), but even then, you need a lot of convenient PODs and storyline setups (very convenient political changes, for starters) to make it work. Second Korean Wars are closer to Asian Fuldapocalypses then they are to "well, it didn't happen, but it could plausibly with a few changes" alternate history conflicts.